The normal anatomic relationship between the retina and the choroid is crucial to eye function. In many ocular diseases or abnormalities such as, for example, chorioretinal degenerations, retinal degenerations, macular degenerations and retinal detachment, the integrity of this relationship is compromised. One of the most common ocular disorders is macular degeneration (MD), a clinical term that is used to describe a variety of diseases that are all characterized by a progressive loss of central vision associated with abnormalities of Bruch's membrane and the retinal pigment epithelium (RPE) (See FIG. 1). These disorders include very common conditions that affect older patients (age-related macular degeneration, or AMD), as well as rarer, earlier-onset dystrophies.
AMD is a significant cause of irreversible blindness worldwide. There is strong evidence that a significant proportion of AMD has a genetic foundation (Gorin et al., MB, Mol Vis. 1999,5:29; Heiba et al., Genet. Epidemiol. 1994; 1:51-67; Seddon et al., Am. J. Ophthalmol. 1997;123:199-206; and Klein et al., Arch. Ophthalmol. 1994;1 12:932-7). Several AMD loci have been identified (Weeks et al., Hum. Mol. Genet. 20009:1329-49; and Klein et al., Arch. Ophthalmol. 1998;116:1082-8). In addition, the ApoE4 allele has been shown to be protective for the disease (Klaver et al., Am J Hum Genet 1998;63:200-6).
The interphotoreceptor matrix (IPM) is an extracellular matrix comprised of an array of proteins, glycoproteins, and proteoglycans, occupying the space between the apical surfaces of the neural retina and the RPE. Hageman and Kuehn (1998), The Pigmented Retinal Epithelium: Current Aspects of Function and Disease, pp. 417-454; Hageman and Johnson (1991), Progr. in Ret. Res. 10:207-209; Hewitt and Adler (1989), Retina 1:57-64; Mieziewska (1996), Microsc. Res. Tech. 15;35:463-471. The IPM is crucial for normal function and viability of retinal photoreceptor cells. It can participates in the exchange of metabolites and catabolic byproducts between the retinal pigment epithelium and photoreceptor cells, the regulation of the subretinal ionic milieu, and the orientation, polarization, and turnover of photoreceptor outer segments (Marmor et al., Arch. Ophthalmol. 1995; 113:232-8; Bok et al., Arch. Ophthalmol. 1993;111:463-71; Hageman et al., Proc. Natl. Acad. Sci. U.S.A 1991;88:6706-6710; Hewitt et al., The retinal pigment epithelium and interphotoreceptor matrix: Structure and specialized function, In: T. Ogden, Editor Retina, St. Louis, Mo., C. V. Mosby Co., 1989, 57-64; and Lazarus et al., Invest. Ophthal. Vis. Sci. 1992,33:364-375). IPM proteoglycans have been shown to mediate photoreceptor cell adhesion (Yao et al., Invest. Ophth. Vis. Sci. 1990,31:2051-2058; Hageman et al., Arch. Ophthalmol. 1995;113:655-660; Marmor et al., Retina 1994;14:181-186; and Hollyfield et al., Retina 1989,9:59-68). IPM 200 and IPM 150 may also mediate photoreceptor cell survival by sequestration of growth factors (Hageman et al., Proc. Natl. Acad. Sci. U.S.A, 1991, 88:6706-6710; and Alberdi et al., Biochem. 1998, 37:10643-52) or through the EGF-like domains contained within their core proteins (Kuehn and Hageman, Mol. Cell Biol. Res. Commun. 1999; 2:103-110; Kuehn and Hageman, Matrix Biology 1999;18:509-518; and Kuehn et al., Mol. Vis. 2000;6:148-56).
Photoreceptor cells are highly vulnerable to dysfunction and/or death in various heritable retinal dystrophies and degenerations (Stone et al., Prog. Retin. Eye Res. 1999;18:689-735). Nucleotide sequence variations in the genes encoding a number of retinal proteins are associated with the etiology of various retinal degenerations. For example, mutations in the genes encoding retinal rhodopsin, beta-phosphodiesterase, rab geranylgeranyl transferase, rim protein, and the RP1 gene product cause retinal degeneration (Dryja et al., Nature 1990;343:364-6; McLaughlin et al., Nat. Genet. 1993;4:130-4; Guillonneau et al., Hum. Mol. Genet. 19998:1541-6; Allikmets, Nat. Genet. 1997;17:122; and Seabra et al., Science 1993;259:377-81).
Cone matrix sheaths (CMSs), distinct domains of the IPM that contain chondroitin 6-sulfate proteoglycan and surround cone photoreceptor cells, have firm attachments to both RPE cells and the neural retina. This adhesive system is sufficiently strong to detach the RPE or tear the CMS following manual separation of the neural retina from the RPE. Hageman et al.(1995), Arch. Ophthalmol. 113:655-660. The role of IPM constituents in mediating retinal adhesion has been investigated following subretinal or intravitreal injections of various enzymes into rabbit eyes and examination of the consequent morphological, biochemical and physiological changes in retinal structure and function. Chondroitinase ABC, neuraminidase and testicular hyaluronidase, three enzymes that degrade oligosaccharides known to be constituents of the IPM, caused diffuse loss of adhesion that is associated with changes in peanut agglutinin (PNA)-binding to CMSs, without affecting photoreceptor function (based on ERG recordings). Yao et al (1990), Invest. Ophthalmol. Vis. Sci. 31:2051-2058; Yao et al. (1992), Invest. Ophthalmol. Vis. Sci. 33:498-503. Enzymatic cleavage of IPM-associated chondroitin sulfate glycosaminoglycans leads to a rapid decrease of retinal adhesiveness in both rabbits and primates in vivo. Yao et al (1990), Invest. Ophthalmol. Vis. Sci. 31:2051-2058; Yao et al. (1994), Invest. Ophthalmol. Vis. Sci. 35:744-748. In addition, disruption of proteoglycan synthesis in vivo leads to loss of CMS-associated chondroitin 6-sulfate proteoglycans, IPM disruption, localized retinal detachments and photoreceptor outer segment degeneration. Lazarus and Hageman (1992), Invest. Ophthalmol. Vis. Sci. 33:364-376. Restoration of retinal adhesion, which recovers steadily between 5 and 20 days following exposure to chondroitinase and neuraminidase, correlates closely with the re-establishment of the normal distribution of PNA-binding glycoconjugates in the IPM. Restoration of impaired adhesion is concomitant with the de novo biosynthesis of IPM chondroitin sulfate proteoglycans. Yao et al. (1992), Invest. Ophthalmol. Vis. Sci. 33:498-503. Thus, it is likely that specific components of the IPM act as major adhesive elements bridging the RPE-retina interface, and that these elements are critically dependent on the metabolic function of RPE and neural retina cells in the microenvironment of the IPM.
Studies in which monkey and human eyes are removed immediately following euthanasia or optic crossclamp, respectively, and the retinas partially “peeled” from the RPE reveal that CMSs remain firmly attached to both the apical RPE and neural retina and elongate up to 4-6 times their normal length in eyes peeled within 45 seconds of enucleation. Additional peeling separation results in separation of the entire RPE from Bruch's membrane, or splitting of the CMSs. These results suggest that adhesion between CMS constituents and the RPE or photoreceptors is stronger than that of the integrity of the CMSs themselves and, as such, provide evidence that CMSs have characteristics consistent with their participation in the establishment and maintenance of retinal attachment. Hageman et al. (1995), Arch. Ophthalmol. 113:655-660; Yao et al. (1990), Invest. Ophthamol. Vis. Sci. 33:498-503.
IPM proteoglycans can be involved in the differentiation and maintenance of specific retinal cells and/or with the establishment of physical interactions between photoreceptor and RPE cells. Recent studies have demonstrated that chondroitin sulfate-containing proteoglycans are first detectable in the mouse IPM a few days prior to the elaboration of photoreceptor outer segments. In human retinas, CMS constituents bound by PNA and the chondroitin 6-sulfate antibody (AC6S) are expressed at a time during development when rudimentary cone outer segments first differentiate. Concentrated accumulations of PNA-binding constituents are observed at 17 to 18 weeks. Chondroitin 6-sulfate is first detected in the IPM between 20 and 23 weeks, when rudimentary cone outer segments begin to differentiate and is solely associated with cone outer segments. At this time, cone photoreceptors are well-polarized and enlarged domes of IPM are associated with them. These studies suggest that there is a staggered expression of PNA- and AC6S-containing IPM constituents, and that the CMSs and IPM constituents can be necessary for subsequent cone outer segment differentiation and survival. This contention is further supported by observations that photoreceptors exhibit some degree of polarity but are unable to maintain differentiated outer segments in culture. In addition, RPE-conditioned medium causes a significant increase in the number of embryonic chick photoreceptor cells forming outer segment-like structures in vitro and stimulate their survival and differentiation.
A number of studies suggest a correlation between changes in IPM composition and the etiology of photoreceptor demise in some degenerations. In the “Royal College of Surgeons” (RCS) rat, or mucopolysaccharidosis VII (MPS VII) mouse, for example, alterations in the distribution of IPM chondroitin 6-sulfate proteoglycan occur prior to photoreceptor degeneration. Chu et al. (1992), Graefes Arch. Clin. Exp. Ophthalmol. 230:476-482; Johnson et al. (1989), Inherited and Environmentally Induced Retinal Degenerations, pp. 217-232; Lazarus et al. (1993), Exp. Eye Res. 54:531-541; Porello and LaVail (1986), Curr. Eye Res. 5:981-993. Similarly, intravitreal injection of β-D-xylopyranoside, a sugar that inhibits GAG chain addition to proteoglycan core proteins, results in cone outer segment degeneration following loss of CMS-associated chondroitin 6-sulfate. These studies strongly indicate that disruption of the normal synthesis or turnover of chondroitin sulfate proteoglycans can lead to cone outer segment degeneration.
The density of rods with abnormal IPM has been shown to increase in correlation with increasing quantities of macular drusen, extracellular deposits in Bruch's membrane associated with AMD. Preliminary analyses of neutral and amino sugars from the IPM of retinas with and without significant numbers of these “PNA-binding rods” show a decrease in IPM sialic acid concentration. In addition, the densities of PNA-binding rods correlate with drusen grade. The incidence of cone degeneration in these same eyes can be as high as 30-40%, in contrast to age-matched controls without significant macular drusen, where cone photoreceptor loss is approximately 12-15%. These studies provide direct evidence that changes in IPM proteoglycan composition are associated with drusen, and that these changes can be related to photoreceptor dysfunction and visual loss that occurs in individuals with AMD.
The vitronectin receptor (VnR) is associated with apical RPE and cone photoreceptor outer and inner segment plasma membranes. VnR co-localizes with components of apposed CMSs at these cell surfaces, and with actin cables within apical RPE microvilli and cone photoreceptor inner segments. Based on the known functions of VnR, it is likely that its interaction with IPM ligands can modulate other cellular activities, such as translation of external cues into signals that affect cytoskeletal organization or modification of other IPM ligands.
Discovery of novel IPM components allows identification of novel therapeutic and diagnostic agents for diseases or conditions associated with abnormal IPM, such as retinal detachment, chorioretinal degenerations, retinal degenerations and macular degenerations such as AMD, or other dystrophies or degenerations involving IPM, cones or rods.